Conventionally, a document reading device is known for reading a code such as characters from a document. The document is composed of bound sheets. The document is, for example, a passport.
In a document reading device, a document is placed on a glass plate or the like, and a surface to be read is irradiated with illumination light to capture an image of the surface to be read. In the case where the document is a passport, for example, the surface to be read is a page carrying a photograph and the passport number. Visible light or infrared light is used as the illumination light.
The visible light is typically white light and is suitable for use in obtaining an image of the surface to be read as in an image scanner. The image is displayed on a monitor, for example.
The infrared light is suitable for reading processing such as optical character recognition. In an infrared image, background images tend to be unclear and the image has high contrast of a code such as characters. This, therefore, allows accurate reading processing.
The visible light and infrared light are an illumination light source and reflection is used. In other words, illumination light such as visible light and infrared light is reflected on the surface to be read and the reflected light is incident upon an imaging unit. In contrast, there is an ultraviolet light source as a light source which does not use reflection.
When an ultraviolet light source is used, shapes are drawn by means of a fluorescent material. When the surface to be read is irradiated with ultraviolet light, the fluorescent material is activated. The imaging unit generates an image with visible light emitted from the fluorescent material. The ultraviolet light source is used to prevent illegal acts such as tampering.
A reading device that uses illumination light is disclosed, for example, in Patent Literature 1. A reading device provided with an ultraviolet light source is disclosed, for example, in Patent Literature 2.
In conventional document reading devices, however, the device size increases when the illumination light source and the ultraviolet light source coexist. In this respect, in recent years, there has been an increasing need for functional improvement of reading devices. If two or more kinds of light sources are to be provided in order to satisfy the need, the space occupied by the light sources increases accordingly. In particular, if both a visible light source and an infrared light source are to be provided as illumination light sources, three light sources are required: visible light source, infrared light source, and ultraviolet light source. This requires even larger space. Further, in recent years, LED elements have been preferred as light sources. If two or more kinds of LED elements are to be arranged in order to provide illumination light and ultraviolet light, the number of elements, and therefore the space occupied by an array of elements, increases accordingly.
In addition, a document such as a passport is likely to curl or lift, which may cause a specular reflection condition as a lighting condition. The specular reflection condition means that the orientation of an image to be captured coincides with the direction of the specular reflection of illumination light. In the specular reflection condition, reflected light may be too strong when an image of the surface to be read is captured. As a result, a so-called “washed-out highlight” appears in the visible light image and a good image cannot be obtained. The “washed-out highlight” is a phenomenon that occurs when light incident upon an imaging device is so strong that the dynamic range of the device is exceeded and a part of image is washed out, leaving a blurred image that is whitish and without an object. In the case of infrared light, contrast of characters and the like similarly decreases and readability decreases when an image is captured in the specular reflection condition. Therefore, it is necessary to avoid the specular reflection condition for illumination light such as visible light and infrared light. In particular, it is important to avoid the specular reflection condition for a passport and the like because many of them are covered with a laminate film or the like and have high reflectance.
Conventionally, the surface to be read is illuminated from the side and an image of the surface to be read is captured directly in front. In this way, the reflection angle (=incident angle) of illumination light can be increased to avoid the specular reflection condition.
However, if deformation of a document such as curl or lift occurs, the angle of the surface to be read may vary and cause the specular reflection condition. Therefore, it is required to be able to avoid the specular reflection condition even if deformation of a document occurs.
To avoid problems described above, a conventional copier or the like is provided with a cover for pressing an object to be read. When the document is a passport or the like, however, it must be read rapidly. Therefore, it is desirable to appropriately illuminate the document without a cover or the like even if the document floats to some extent.